nimeshrmr

<a href="http://profileplugin.com/invitation-codes-addon/" rel="nofollow ugc">Invitation Codes for User Profiles Made Easy</a> is a free addon provided for limiting the registration only to invited users. Admin can enable/disable invitation code based registration anytime. Once enabled, users without valid invitation code can?t create a new registration.
Please note that this is an addon for <a href="https://goo.gl/Dtqxsx" rel="nofollow ugc">User Profiles Made Easy</a> plugin and you need at least UPME version 2.1.10 to use it.
<blockquote>
Main Features
<ul>
<li>Create and manage invitation codes and quota</li>
<li>Send invitations to emails from WordPress backend</li>
<li>Limit registration to invited users</li>
</ul>
</blockquote>

Invitation Codes for User Profiles Made Easy

Limit WordPress User Registration to Invited Users with User Profiles Made Easy.

Peshmerge Morad

This plugin is created to enable WordPress site owners to specify a custom invitation code to be used with Gravity Forms. 
Once an invitation code is specified on a form, the user can’t submit that form until that invitation code is entered in 
the field during the form submission. This plugin can be combined with the Gravity Forms User Registration Add-On to make 
sure that only people with a correct invitation code can register on your website.

Invitation Codes: Gravityforms Add-on

A GravityForms addon to enable users to use custom invitation codes in combination with GravityForms.

Invitation Codes for User Profiles Made Easy vs Invitation Codes: Gravityforms Add-on